In a game system which makes a player, for example, step on panels of a foot panel to a tune, it has been already well-known that some marks are displayed to inform a player of the timing to step on the panel and which panel to be stepped on, and some predetermined game background image is displayed behind the marks. On the other hand, in a karaoke system where some background image is displayed with lyrics on a monitor, it has been already known that the method to add modification effects which differ from tune to tune by using small amount of image data (for example, laid-open application publication JP07-271386) and the method to make various types of background image by switching or inserting limited pieces of image data prepared in advance (for example, laid-open application publications JP05-232978, JP03-288193, JP05-323980).